Shigaite

Shigaite
Reddish-brown shigaite crystal (2 cm across) with pink rhodochrosite from South Africa
General
CategorySulfate mineral
Formula
(repeating unit)		NaAl3(Mn2+)6(SO4)2(OH)18·12H2O[1]
IMA symbolSga[2]
Strunz classification7.DD.35[3]
Dana classification31.1.2.1[3]
Crystal systemTrigonal
Crystal classRhombohedral (3)
H-M symbol: (3)[3]
Space groupR3[4]
Unit cella = 9.51 Å, c = 32.83 Å,[3] Z = 3[5]
Identification
ColorYellow, burnt orange, brown, black[5]
TwinningOn {0001}[5]
CleavagePerfect on {0001}[5]
TenacityModerately flexible[5]
Mohs scale hardness2[5]
LusterVitreous to dull[3]
StreakVery pale yellow to white[5]
DiaphaneityTransparent[3]
Specific gravity2.32[4]
Optical propertiesUniaxial (−)[3]
Refractive indexn = 1.546[4]
PleochroismDistinct; O = yellow; E = very pale yellow[5]
Ultraviolet fluorescenceNon-fluorescent[4]

Shigaite is a mineral with formula NaAl3(Mn2+)6(SO4)2(OH)18·12H2O that typically occurs as small, hexagonal crystals or thin coatings. It is named for Shiga Prefecture, Japan, where it was discovered in 1985.[3] The formula was significantly revised in 1996, identifying sodium as a previously unknown constituent.

Description

Dark-red and yellow shigaite on pink rhodochrosite

Shigaite occurs as hexagonal tabular crystals up to 2 cm (0.79 in) in size or as thin films and coatings. The mineral can be yellow, burnt orange, brown or black in color.[5] Shigaite occurs in metamorphosed deposits of manganese ore[5] and is the Mn2+ analogue of motukoreaite.[6]

Structure

Shigaite consists of oxycation sheets of [AlMn2+2(OH)6]1+ intercalated with oxyanion sheets of [Na(H2O)6{H2O}6(SO4)2]3−. Linkage between the sheets and within the oxyanion sheet results largely through hydrogen bonding.[6]

History

Shigaite was discovered in 1985 in the Ioi Mine,[a] Shiga Prefecture, Japan.[3] The original study, published in the journal Neues Jahrbuch für Mineralogie, Monatshefte,[1] identified the formula as Al4Mn7(SO4)2(OH)22·8H2O.[7] The formula was significantly revised in 1996 using a sample from the N'Chwaning Mine, South Africa.[6] Sodium, discovered to be a component of shigaite, was not identified in the original study. However, an unidentified volatile had been noted that presumably was a sodium-containing complex.[8]

Distribution

As of 2012, shigaite is known from the following sites:[3]

  • Iron Monarch open cut, South Australia, Australia
  • Poudrette quarry, Quebec, Canada
  • Ioi mine, Shiga Prefecture, Japan
  • Wessels Mine, Northern Cape Province, South Africa
  • N'Chwaning Mine, Northern Cape Province, South Africa
  • Homer Mine, Michigan, United States
  • Bengal Mine, Michigan, United States

The type material is housed in the National Museum of Natural History in Washington, D.C. as sample 122089.[5]

Association

Shigaite has been found associated with the following minerals:[5]

Notes

  1. ^ Some sources incorrectly list it as the Loi Mine,[7] presumably because of a mistaken reading of uppercase "i" as lowercase "L".

References

Wikimedia Commons has media related to Shigaite.
  1. ^ a b Nickel, Ernest H. "IMA/CNMNC List of Mineral Names" (PDF). Materials Data, Inc. Retrieved May 6, 2012.
  2. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  3. ^ a b c d e f g h i j "Shigaite". Mindat. Retrieved April 20, 2012.
  4. ^ a b c d "Shigaite Mineral Data". Webmineral. Retrieved May 20, 2012.
  5. ^ a b c d e f g h i j k l "Shigaite" (PDF). Handbook of Mineralogy. Mineral Data Publishing. Retrieved April 20, 2012.
  6. ^ a b c Cooper, p. 91.
  7. ^ a b Hawthorne, Frank C.; et al. (November–December 1986). "New Mineral Names" (PDF). American Mineralogist. 71 (11 & 12): 1546. Retrieved May 17, 2012.
  8. ^ Cooper, p. 96.

Bibliography

Further reading